The present invention relates to an image forming apparatus such as a copying machine, a facsimile, a printer, and so on and, more particularly, an image fixing device using the electromagnetic induction.
The market needs for energy saving and higher speed on the image forming apparatus such as the copying machine, the facsimile, the printer, and the like are growing nowadays. In order to achieve these demanded performances, it is important to improve a thermal efficiency of the fixing device used in the image forming apparatus.
In the image forming apparatus, according to the image forming process such as electrophotographic recording, electrostatic recording, magnetic recording, or the like, the unfixed toner image is formed on the recording medium such as sheet member, printing paper, photosensitive paper, electrostatic recording paper, or the like by the image transfer process or the direct process. As the fixing device for fixing the unfixed toner image, the fixing device using the convective heating process such as heat roller process, film heating process, electromagnetic induction heating process, or the like is widely used.
As the fixing device using the electromagnetic induction heating process, in JP-A-8-22206, the technology to generate the Joule's heat by the eddy current, which is generated in the heat generating member made of the magnetic metal member by the magnetic field generated by the induction heating means made of the exciting coil, to cause the heat generating member to generate a heat based on the electromagnetic induction is proposed.
A configuration of the fixing device using the electromagnetic induction heating process in the prior art will be explained hereunder. Here, FIG. 18 is a view showing a fixing device using the electromagnetic induction heating process in the prior art.
As shown in FIG. 18, the fixing device is constructed by an exciting coil unit 1001 consisting of a ferrite core 1001a and an exciting coil 1001b, a heating roller 1002 made of magnetic metal member, a fixing roller 1003 having an elastic layer as a surface layer, a fixing belt 1004 stretched by the heating roller 1002 and the fixing roller 1003 and having a release layer as a surface layer, and a pressing roller 1005 opposed to the fixing roller 1003 to press it. The nip portion is formed between the fixing roller 1003 and the pressing roller 1005. The heating roller 1002, the fixing roller 1003, and the fixing belt 1004 are rotated and moved in a clockwise direction, as indicated by an arrow D, by a driving means (not shown). The pressing roller 1005 is driven by the driving means (not shown) so that it is rotated and moved in a counterclockwise direction D2. i.e. in a direction opposite to the direction indicated by an arrow D.
According to the result calculated by a software to maintain the fixing belt 1004 at a predetermined temperature based on temperature information sensed by a temperature sensing means 1006, a current is supplied to the exciting coil 1001b from an inverter circuit (not shown) to generate an alternating magnetic field (not shown). The alternating magnetic field generated by the exciting coil 1001b generates the eddy current in the heating roller 1002. Then, this eddy current is converted into a heat (Joule's heat) due to the electric resistance of the heating roller 1002 to cause the heating roller 1002 to generate the heat, so that the fixing belt 1004 is heated.
In the situation that the temperature of the fixing belt 1004 rises to a predetermined temperature, when a recording member 1007 on which unfixed toner images 1008 are formed by an image forming portion (not shown) is introduced into the nip portion formed by between the fixing roller 1003 and the pressing roller 1005, such recording member 1007 is carried into the fixing nip portion while being put between the fixing belt 1004 and the pressing roller 1005 and as a result the unfixed toner images 1008 on the recording member 1007 are fused and fixed onto the recording member 1007.
Also, the heating roller 1002, the fixing roller 1003, the fixing belt 1004, the pressing roller 1005, and the temperature sensing means 1006 are constructed as one fixing unit 1009. Thus, the user can exchange the fixing unit when such unit comes to the end of lifetime.
In this case, the recording member 1007 is separated from a surface of the fixing belt 1004 when it passes through the exit of the fixing nip portion, and then carried into a paper discharge tray (not shown).
According to the fixing device constructed as above, a heat generating efficiency can be improved and a warm-up can be further shortened.
In such fixing device using the electromagnetic induction heating process, the heating portion has a good heat generating efficiency, nevertheless the conventional software applied to control the temperature of the fixing device lacks the responsibility and the reliability to follow the very quick temperature rise. Therefore, there exists the problems such that the fixing device can not satisfy required specifications for the image fixing, the fixing device lacks the safety due to the delay in the response to sense the abnormality, and so forth.
Further, the necessity to satisfy the market need for a higher operation speed arises quickly and for that purpose the fixing device must be heated in the standby state (this mode is referred to as a “pre-heating mode” hereinafter) other than the image forming operation. Therefore, there existed the problem that such request cannot be sufficiently satisfied by the conventional temperature control of the fixing device.